Most modern processing systems, including personal computers (PCs), rely on one form or another of optical data storage. For example, CD-ROM drives are now standard equipment on nearly all new PCs. Nearly all multimedia software, including video games, maps, encyclopedias, and the like, are sold on CD-ROM. Also, compact discs are the most prevalent storage medium for musical recording. More recently, digital video disc (DVD) technology has been introduced that will expand the storage capacity of standard CD technology from about one-half gigabyte to about five gigabytes.
The large storage capacities and relatively low costs of CD-ROMs and DVDs have created an even greater demand for still larger and cheaper optical storage media. Many large businesses rely on jukebox-style CD changers in order to access a particular one of potentially hundreds of discs. Motion pictures released in optical storage format still require multiple CDS or oversized laser discs. However, it appears that the limits of CD-ROM and DVD technology are being reached. In order to continue to improve the capacity and speed of optical storage systems, research increasingly focuses on holographic storage devices capable of storing hundreds of gigabytes in a CD-sized storage medium.
A number of holographic data storage systems have been developed that are capable of storing and retrieving an entire page of data at a time. In these systems, data to be stored is first encoded in a two dimensional (2D) optical array, for example on a liquid crystal display (LCD) screen, which is one type of spatial light modulator (SLM). Another type of SLM is Texas Instruments' Digital Mirror Device, which is a reflective device that allows the reflectivity of each pixel to be changed. The term "SLM" also includes fixed masks of varying optical density, phase, or reflectivity.
A first laser beam, called an object beam, is transmitted through the SLM and picks up an intensity pattern from the data squares and rectangles (pixels) in the 2D array. The data-encoded object beam is ultimately projected onto and into a light-sensitive material, called a holographic memory cell (HMC). A second laser beam, called a reference beam, is also projected onto and into the holographic memory cell. The object beam and the reference beam then cross to produce an interference pattern throughout a volume element of the HMC. This unique interference pattern induces material alterations in the HMC that generate a hologram.
The formation of the hologram in the holographic memory cell is a function of the relative amplitudes of, and the phase differences between, the object beam and the reference beam. It is also highly dependent on the incident angles at which the object beam and the reference beam were projected onto the holographic memory cell. After hologram storage, the data beam may be reconstructed by projecting into the HMC a reference beam that is the same as the reference beam that produced the hologram. The hologram and the reference beam then interact to reproduce the data-encoded object beam, which may then be projected onto a two-dimensional array of light sensitive detectors which read back the data by sensing the pattern of light and dark pixels.
The object beam produced by the spatial light modulator has a high space-bandwidth product (SBP). The SBP of a beam is equal to the number of pixels the beam contains. For example, the 800.times.600 pixel image produced by a SVGA computer monitor has a SBP of 480,000. When high SBP beams are projected into a holographic memory cell, it is important to keep the optical path lengths traversed by the beams constant. Otherwise, the high SBP image will go out of focus and the data will be lost.
Maintaining a constant optical path length in order to keep the high SBP image of the object beam in focus necessarily makes it difficult to steer the object beam to different areas on the surface of the holographic memory cell, because such steering frequently causes the optical path length to change. For this reason, many holographic data storage systems project a reference beam through an acousto-optic cell, which diffracts the reference beam through an optical system, such as a 4-f imaging system, that has a fixed optical path length. Altering the frequency of the acoustic wave changes the angle at which the reference beam is diffracted and therefore incident to the surface of the holographic memory cell. Systems using such angle-tuned reference beam steering are known as "angle multiplexing" systems and are distinguished by their capability to project different pages of data into the same location on the surface of the holographic memory cell, but at different angles of reference-beam incidence. The data is then retrieved by steering the interrogating reference beam at different angles of incidence. However, these prior art systems are inadequate to steer a high SBP object beam to different areas of the holographic memory cell because of their inherent limitations with respect to space-bandwidth product throughput.
Accordingly, there is a need in the art for improved optical systems that are capable of steering high space-bandwidth product images to different regions on the surface of a holographic memory cell without causing the image to lose focus. There is a further need in the art for improved optical systems capable of steering high space-bandwidth product images in more than one dimension in a coordinate system. There is a still further need in the art for improved optical systems capable of steering complex reference beams in more than one dimension in a coordinate system.